1. Field of the Invention
In many industrial operations it is desirable to known the dissolved oxygen content of various aqueous streams. To illustrate, the direct measurement of the dissolved oxygen concentration is often needed in control of process variables of industrial aeration treatment of waste effluents. Direct measurement of dissolved oxygen content of surface water and streams is often desired in environmental studies on marine life. In addition, differential measurement of oxygen concentrations of various aqueous media between the inlet and outlet of a flow-through system can be used to monitor the activity of aerobic microbial processes.
Measurements of dissolved oxygen content are often desired at remote sites and it is desirable to have an instrument having a long cell life without need for frequent recalibration or standardization and free from frequent maintenance. Furthermore, what maintenance may be required should be free of mechanical complexity, thereby permitting field servicing of the instrument and obviating the need to return the instrument to a shop.
2. Description of the Prior Art
U.S. Pat. Nos. 2,913,386 and 2,651,612 describe dissolved oxygen analyzing instruments which employ a conductive electrolyte and electrodes of metals having sufficient electromotive dissimilarities to provide electrochemical cells having output voltages which are a function of dissolved oxygen concentration. These instruments are shown to have an oxygen permeable membrane isolating the cell from the liquid being analyzed. The typical reaction of the cells is: EQU Cathode: O.sub.2 + 2H.sub.2 O + 4e .fwdarw. 4OH.sup.- EQU Anode: 4OH.sup.- + 2M .fwdarw. 2MO + 2H.sub.2 O + 4e .sup.- EQU Overall: 2M + O.sub.2 .fwdarw. 2MO
typically a noble metal such as silver, platinum, palladiun or gold was used for the cathode and a metal such as copper, lead, nickel, cadium or tin is used for an anode.
A common failing of these dissolved oxygen instruments is their inability to maintain uniform activity over extended periods of use. As indicated in the preceding reaction, the cell operation forms a metal oxide. Metal oxides have relatively low solubility in aqueous media and, accordingly, rapid polarization of the anode by a coating of the insoluble metal oxide can occur. To minimize the requirement for frequent restandardization of the cell, it is desirable that the electrical conductivity through the cell, i.e. through the electrolyte as well as across the electrode-electrolyte interfaces be stable and relatively independent of the reactions occurring at the electrodes. It is, therefore, desirable to provide a cell having an electrolyte in which the metal oxide is relatively soluble to insure removal of the metal oxide from the anode surface before its precipitation. Additionally the electrolyte should have a sufficiently high concentration of mobile, current-conducting ions to provide a high electro-chemical conductivity and thereby insure maximum instrument sensitivity. Heretofore, these electrolyte characteristics have not been optimized and either or both of the electrical conductivity or metal oxide solubility of the electrolyte have been compromised.
When the instrument is used over an extended period of time, the concentration of the metal oxide in the electrolyte reaches its limit of solubility and precipitates of metal oxides are formed. To insure maximum life of the instrument, the electrodes should be protected from becoming covered with the precipitates which are formed in the electrolyte. Since the surface area of the cathode is minimized in these cells, it is particularly critical that the cathode be shielded from such precipitation if operation for extended periods is to be achieved.
The electro-chemical cell is shielded from the liquid under investigation by a protective oxygen-permeable barrier that is located adjacent to the cathode. The barrier is fragile and must be supported by a durable backing to permit its use in liquids which are subjected to pressure surges or which contain suspended solid matter that may strike the membrane. In many prior devices, the cathode and membrane are relatively fragile and are not well suited for such applications. Additionally, a common failing of the prior devices is that the cathode or protective membrane are not readily accessible and their replacement requires extensive shop maintenance
The instrument of this invention obviates most of the aforementioned failings of the prior art devices and provides a constant sensitivity of measurement of dissolved oxygen in aqueous media over extended periods of use.